There have been proposed several polymer aqueous dispersions as conductivity-imparting agents or antistatics with which an electrically insulating substrate, such as paper and plastic films, is impregnated or coated, followed by drying. Among them, well known is an aqueous dispersion of a cationic styrene polymer obtained by reacting chloromethylated polystyrene with a tertiary amine to form a quaternary salt, as disclosed, e.g., in Japanese Patent Publication Nos. 7871/73 and 34150/74. However, since a coating film formed by coating the cationic styrene polymer on paper or a plastic film and drying lacks softness, the substrate tends to curl or the coating film tends to suffer cracking during preservation, causing adverse influences on surface conductivity.
In cases where a cationic polymer is used in a conductive layer of an electrographic recording material, it is generally mixed with a metal oxide semi-conductor, etc. for the purpose of improving conductivity and reducing humidity dependence of surface resistivity as taught, e.g., in Japanese Patent Application (OPI) Nos. 9524/80 and 33133/80 (the term "OPI" as used herein means "unexamined published Japanese patent application"). In this connection, the cationic styrene polymer, when used alone, is insufficient in terms of receptivity to inorganic fillers. The receptivity to the metal oxide semi-conductor is of particular importance taking stability of a recorded image in a low humidity atmosphere into consideration.
On the other hand, well-known aqueous dispersions of a cationic polymer prepared from an ethylene copolymer include an aqueous dispersion obtained by reacting an ethylene/aminoalkyl acrylate compound copolymer with an acid in water as disclosed in Japanese Patent Application (OPI) No. 16542/72. This aqueous dispersion is excellent in receptivity to inorganic fillers, such as metal oxide semi-conductors (e.g., zinc oxide, tin oxide, titanium oxide, etc.) and, when coated on a substrate, e.g., paper and plastic films, and dried, provides a semi-conductive coating film having not only softness but resistance to water or solvents. In spite of these advantages, however, the coating film formed by the aqueous dispersion prepared from an ethylene/aminoalkyl acrylate compound copolymer has a relatively higher surface resistivity as compared with that of the cationic styrene polymer Such an aqueous dispersion applied to a conductive layer of an electrographic recording material fails to attain sufficient recording performance and, therefore, has not yet been put in practical use.
In the field of sheet forming, the system disclosed in Japanese Patent Application (OPI) Nos. 98304/83 and 180697/83 is noteworthy. According to this system, sizing and wet strength enhancement can be achieved simultaneously by adding to an aqueous pulp slurry an aqueous dispersion obtained by reacting an ethylene/ aminoalkyl acrylate compound copolymer with a variety of organic or inorganic acids in water to form a quaternary salt and then reacting it with an epihalohydrin through addition reaction.
The inventors have conducted studies toward practical application of the above-described technique. However, since the aqueous dispersion prepared from the ethylene/aminoalkyl acrylate compound copolymer should be added in a relatively large amount in order to produce effects as expected, the results obtained were unsatisfactory for practical use from the standpoint of economy as compared with the commonly employed system wherein a sizing agent and a wet strength agent are used in combination.
Hence, it has been demanded to solve various problems associated with the conventional aqueous dispersion prepared from an ethylene/aminoalkyl acrylate compound copolymer, i.e., insufficient conductivity of a coating film obtained therefrom, and insufficient performance to simultaneously impart sizing properties and wet strength in sheet forming.